He would leave her, but he'd certainly get back to her because he'd need her around. Second choice or not, he does love her, and although he wishes she'd just forget Grigorij, he'd be quite content go be by her side, especially after the kids are gone.
Indeed, she really did love Nikolaj as a baby, being her first born, and Nikolaj would love her back like a child would, but it just didn't last very long on his part, and nobody could blame him for that.
He'd just try his best to get back in touch with her, but she'd be either violent or apathetic towards him, and he'd hate it. He just grew up hating what she did and her herself, and he wouldn't change just because his mom would tell him she still likes him.